Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session N59: Noninteracting Topological Insulators and Topological Crystalline Insulators in SolidState MaterialsRecordings Available

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Chair: Benjamin Wieder, Massachusetts Institute of Technology Room: Hyatt Regency Hotel DuSable AB 
Wednesday, March 16, 2022 11:30AM  11:42AM 
N59.00001: High SpinChernNumber Insulator in αAntimonene Baokai Wang, XIAOTING ZHOU, YenChuan Lin, Hsin Lin, Arun Bansil We report on a new topological phase characterized by a highnumber quantized Spin Hall conductivity in αSb, which we have found in investigating the electronic structures of monolayer αphase group V elements. This distinct topological phase is invisible in the symmetrybased topological quantum chemistry (TQC) and symmetry indicators (SIs). Since αAs and Sb share the same band representations at highsymmetry points, they are both trivial insulators in terms of TQC and SIs. However, we demonstrate that there is a topological phase transition between As and Sb that involves a bandgap closing at two kpoints on the highsymmetry line XΓX. In the absence of spinorbit coupling (SOC), As is a trivial insulator, while Sb is a Dirac semimetal with four Dirac points (DPs) located away from the highsymmetry lines. Inclusion of S_{z}conserved SOC gaps out the DPs and induces a nontrivial Berry curvature and drives Sb into the new highspin Chern number topological phase. The band structure of αBi differs from that of Sb by a band inversion at Γ, transforming Bi into a Z_{2} topological insulator. Our study shows that quantized spin Hall conduc tivity can serve as a topological invariant beyond the Z_{2} topological invariant for characterizing the topological phases. 
Wednesday, March 16, 2022 11:42AM  11:54AM 
N59.00002: Transport measurements in Porous Bi_{2}Te_{3} thin films Golrokh Akhgar, Alexander Nguyen, David Cortie, Abuduliken Bake, Weiyao Zhao, Chang Liu, Michael S Fuhrer, Dimitrie Culcer, Alex R Hamilton, Mark T Edmonds, Julie Karel Recent theoretical work has predicted the existence of disordered topological insulators [1, 2], however, minimal experimental work has been conducted on disordered TIs. Here we used molecularbeam epitaxy (MBE) to grow Bi_{2}Te_{3} thin films that were comprised of nanocrystals embedded in an amorphous matrix. Further disorder was introduced through Ne ion irradiation which produced porosity in the films. 
Wednesday, March 16, 2022 11:54AM  12:06PM 
N59.00003: Persistence of symmetryprotected Dirac points at the surface of the topological crystalline insulator SnTe Olga Arroyo Gascón, Yuriko C Baba, Jorge I Cerdá, Oscar de Abril, Ruth Martínez, Francisco DominguezAdame, Leonor Chico We investigate the effect of a nonmagnetic donor impurity located at the surface of the SnTe topological crystalline insulator. In particular, the changes on the surface states due to a Sb impurity atom are analyzed by means of abinitio simulations of pristine and impuritydoped SnTe. Both semiinfinite and slab geometries are considered within the firstprinciples approach. Furthermore, minimal and Green's function continuum models are proposed with the same goal. We find that the Dirac cones are shifted down in energy upon doping; this shift strongly depends on the position of the impurity with respect to the surface. In addition, we observe that the width of the impurity band presents an evenodd behavior by varying the position of the impurity. This behavior is related to the position of the nodes of the wave function with respect to the surface, and hence it is a manifestation of confinement effects. We compare slab and semiinfinite geometries within the abinitio approach, demonstrating that the surface states remain gapless and their spin textures are unaltered in the doped semiinfinite system. In the slab geometry, a gap opens due to hybridization of the states localized at opposite surfaces. Finally, by means of a continuum model, we extrapolate our results to arbitrary positions of the impurity, clearly showing a nonmonotonic behavior of the Dirac cone. 
Wednesday, March 16, 2022 12:06PM  12:18PM 
N59.00004: Resolving the topological classification of bismuth with topological defects Nurit Avraham Bismuth, due to its large spinorbit coupling, plays a fundamental role in many topological materials. Yet the topological classification of pure Bismuth has remained, thus far, rather ambiguous. While some theoretical models indicate its trivial topological nature, other theoretical and experimental studies suggest nontrivial topological classifications, such as a strong or a higher order topological insulator. I will explain the origin for this ambiguity and present scanning tunneling microscopy and spectroscopy data in which we resolve the topological classification of Bismuth, as a strong topological insulator with weak indices, by spectroscopically mapping the response of its boundary modes to a topological defect in the form of a screw dislocation [1]. 
Wednesday, March 16, 2022 12:18PM  12:30PM 
N59.00005: Derivation of effective Hamiltonian for Na_{2}XY (X=Mg,Cd;Y=Pb,Sn) dual topological insulators via k.p theory Warlley H Campos, Poliana H Penteado, Denis R Candido, Carlos Egues Dual topological insulators (DTIs) are characterized by spin and mirror Chern numbers – stemming from time reversal and crystalline symmetries, respectively – that make them more robust as compared to ordinary TIs. Here, we apply the k.p method [1] and theory of invariants [2] to derive an effective Hamiltonian for Na_{2}XY (X=Mg,Cd;Y=Pb,Sn) materials, predicted to be quasi2D DTIs by abinitio calculations. The next step is to calculate the topological invariants and investigate the behavior of the edge states and their robustness in the presence of an external magnetic field. Our study is important for a better understanding of DTIs, specially Na_{2}XY, and their potential applications. The authors acknowledge Julian Zanon for helpful discussions. 
Wednesday, March 16, 2022 12:30PM  12:42PM 
N59.00006: Magnetic field induced charge symmetry in the bulk of the topological insulators Bi_{2}Se_{3} and Bi_{2}Te_{3 }investigated with NMR Robin Guehne Experiments on 3dimensional topological insulators focus on the characterization of the special, gapless surface states that emerge as a consequence of the bulk energy band inversion. The bulk states, however, are important, as well, as they provide a direct access to the topologically nontrivial band structure. For example, with nuclear magnetic resonance (NMR) we were able to measure the realspace fingerprint of the bulk energy band inversion, i.e. the redistribution of charges in the chemical structure, in the model topological insulator Bi_{2}Se_{3} [1]. In this system, free carriers as induced by selfdoping play a key role because they populate those bands subject to the band inversion. Our measurements further reveal that these conducting electrons possess special properties governed by a strong spinorbit interaction [13]. We show on the basis of a comprehensive experimental evidence from ^{209}Bi NMR in Bi_{2}Se_{3} and Bi_{2}Te_{3} how the local charge symmetry at the nuclear site apparently follows the external magnetic field. This behavior may constitute a so far undocumented rotational degree of freedom of strongly spinorbit coupled conduction electrons with partially unquenched orbital angular momentum. 
Wednesday, March 16, 2022 12:42PM  12:54PM 
N59.00007: Evidence for 2D conduction channels in hydrogenated Bi_{2}Te_{3} Ayesha Lakra, Entela Buzi, Haiming Deng, Lukas Zhao, Kyungwha Park, Lia KrusinElbaum Topological insulators are quantum solids with metallic surface states that have Dirac band structure and are immune to backscattering by the nonmagnetic disorder. However, ubiquitous charged bulk defects pull the Fermi energy into the bulk, denying access to surface charge transport. We demonstrate that by inserting/removing ionic hydrogen H^{+} in a bulk Bi_{2}Te_{3} crystal, with hole densities in the 10^{20} cm^{3} range, these defects can be compensated, moving the Fermi level across the charge neutral point (CNP) into the bulk gap. The magnetoresistance R_{xx}(H) evolves from a quadratic field dependence of a typical bulk metal into a weak antilocalization (WAL) regime with a characteristic lowfield cusp near the CNP. From the fits to weak localization theory we obtain the cusp parameter α≅1.004 ± 0.005, corresponding to two 2D quantum conduction channels supported by top and bottom surfaces. The obtained temperature dependence of the dephasing length l_{φ }∝1/√T is characteristic of the 2D quantum interference. The 2D character is further confirmed by the scaling of R_{xx}(H) with the outofplane field component H⊥= Hcos??. Our results show that the 2D quantum transport can be accessed by hydrogenation disregarding the 3D bulk size. 
Wednesday, March 16, 2022 12:54PM  1:06PM 
N59.00008: Tunable emergent topological surface states in Sb/Bi_{2}Te_{3} and Bi_{2}Te_{3}/Sb thinfilm heterostructures Yao Li, John W Bowers, Joseph A Hlevyack, MengKai Lin, TaiChang Chiang Tuning the Dirac surface states of topological insulators (TIs) to achieve desirable surface properties is crucial for developing advanced TIbased electronics, but readily tunable TIs are limited. Our study of synthesized Sb/Bi_{2}Te_{3 }and the reversed Bi_{2}Te_{3}/Sb topological thinfilm heterostructures, reported herein, illustrates a way to substantially broaden the class of tunable systems. In bulk form, Sb (a semimetal) and Bi_{2}Te_{3} (an insulator) both host topological surface states with the invariant Z_{2} = –1, whereas ultrathin Sb and Bi_{2}Te_{3} films by themselves are fully gapped insulators. Photoemission band mappings, together with theoretical simulations of the band structure, reveal that in the singlelayer Sb limit, the Sb/Bi_{2}Te_{3} heterostructure supports emergent topological surface states strongly localized at the Sb surface. Further Sb coverage leads to a thicknessmediated evolution of the topological surface states along with a shift in the Dirac point energy. This heterostructure thus function as a TI with tunable surface properties controlled by Sb film thickness. These results and related data from the complementary system Bi_{2}Te_{3}/Sb will be presented. 
Wednesday, March 16, 2022 1:06PM  1:18PM Withdrawn 
N59.00009: Surface transport anomalies of quasionedimensional weak topological insulators Tianyi Xu, Fengcheng Wu, Fan Zhang Prototypical weak topological insulators (WTI) have been theoretically predicted to be realized in quasionedimensional materials Bi4X4 (X = Br, I) [PRL 116, 066801 (2016)] and then experimentally confirmed via ARPES [PRX 11, 031042 (2021) & Nature 566, 518 (2019)]. The unique surface states of such a WTI have two entangled Dirac cones with strong anisotropy. We study theoretically the integer quantum Hall effect of such WTI surface states and show the important roles played by the special geometry, symmetry, topology, and their interplay in Bi4X4. We also predict prominent signatures in transport experiments. 
Wednesday, March 16, 2022 1:18PM  1:30PM 
N59.00010: Persistent negative THz photoconductivity in the vertical topological pn junction Sb_{2}Te_{3}/Bi_{2}Te_{3} Yinchuan Lv, Fahad Mahmood, Soorya Suresh, James N Eckstein A thinfilm heterostructure of Sb_{2}Te_{3 }and Bi_{2}Te_{3} can function as a vertical topological pn junction with a layerdependent Fermi level. The spatial separation of the intrinsically p and n doped topological surface states makes this structure a promising candidate for realizing a topological exciton condensate. Here we perform infrared pumpTHz probe measurements on epitaxial Sb_{2}Te_{3}/Bi_{2}Te_{3} heterostructures to track the real and imaginary parts of the optical conductivity in response to photoexcitation. After the initial free carrier excitation and decay on the order of a few ps, a persistent negative photoconductivity (NPC) is observed over a time > 1 ns. The NPC shows a strong temperature dependence and is only present below 100 K. We will discuss various sources of the NPC in terms of possible exciton formation, lattice heating or scattering between the topological surface states and localized defect states. 
Wednesday, March 16, 2022 1:30PM  1:42PM 
N59.00011: Resonant tunneling between Landau levels formed in topological insulator surfaces realized in epitaxially grown van der Waals heterostructures Joon Young Park, Young Jae Shin, Jeacheol Shin, Jehyun Kim, Janghyun Jo, Hyobin Yoo, Danial Haie Najafabadi, Robert M Huber, Arijit Gupta, Kenji Watanabe, Takashi Taniguchi, Wan Kyu Park, Miyoung Kim, Dohun Kim, GyuChul Yi, Philip Kim Tunneling spectroscopy in van der Waals heterostructures with highly crystalline and atomically sharp interfaces provides a powerful tool to probe the density of states of the materials and the selection rule of tunneling electrons. Here, we present the fabrication and electric transport characterization of vertical tunnel junctions made of topological insulators (TIs) separated by an afewatomthick hBN tunnel barrier. We grow Bi_{2}Se_{3} and Sb_{2}Te_{3}, which are n– and p–type TIs, respectively, on both sides of suspended ultrathin hBN layers using molecular beam epitaxy. Our highresolution transmission electron microscopy reveals an atomically abrupt and epitaxial interface between the hBN substrate and the top and bottomTIs. We perform fieldangledependent magnetotunneling spectroscopy on the junctions and observe energymomentumspin resonance of Dirac electrons tunneling between helical Landau levels developed in the topological surface states. 
Wednesday, March 16, 2022 1:42PM  1:54PM 
N59.00012: Quantum transport in patterned SnTe Shuhang Pan, Stephen D Albright, Frederick J Walker, Charles H Ahn SnTe belongs to the class of topological crystalline insulators (TCI), which have nontrivial conducting states where the mirror symmetry is broken at surfaces and edges. To study this behavior, we synthesize SnTe films using molecular beam epitaxy (MBE). The films have high surface area and edge length to volume ratios, which is achieved by growing films patterned with a semiregular array of square vacancies (200 nm x 200 nm). To measure the effect of these vacancies on electronic conduction, we compare quantum transport measurements on both patterned and unpatterned films. The magnetoconductivity results are fitted with the HikamiLarkinNagaoka (HLN) equation and reveal a mixture of weak localization and weak antilocalization (WL/WAL), which indicates conduction in spinmomentum locked electronic states. The stronger WAL effect at higher temperatures (5K to 10K) in the patterned films shows the suppression of bulk conduction by the presence of square vacancies, and the limited coherence length at low temperatures (2K to 5K) is consistent with scattering at the vacancy boundaries. 
Wednesday, March 16, 2022 1:54PM  2:06PM 
N59.00013: Bi_{4}Br_{4}based hightemperature quantum spin Hall physics Chiho Yoon, Yanfeng Zhou, Hongki Min, Fan Zhang The manifestation of quantum phenomena at room temperatures is a major challenge but also a major goal in physics, and this is particularly true for the quantum spin Hall effect. Here we report the theoretical demonstration of a variety of hightemperature (and in some cases even roomtemperature) quantum spin Hall systems based on Bi_{4}Br_{4}, a rare higherorder topological insulator identified recently. We also common on the key experimental studies. 
Wednesday, March 16, 2022 2:06PM  2:18PM 
N59.00014: Nanofabrication and transport studies of Quasionedimensional Topological Insulators Zheneng Zhang, Yulu Liu, Ruoyu Chen, Marc Bockrath, Chun Ning Lau, Yanfeng Zhou, Chiho Yoon, Sheng Li, Xiaoyuan Liu, Bing Lv, Fan Zhang, Nikhil Dhale Quasionedimensional (1D) topological insulators (TIs) promise advantages over their threedimensional (3D) and quasitwodimensional (2D) counterparts, such as multiple cleavage planes, straininduced phase transitions between weak TI, strong TI, and trivial insulator, and hosting of prototype higherorder TIs with helical hinge modes. Prototypical examples of quasi1D TIs are Bi4I4 and Bi4Br4, which are composed of a periodic stack of atomic chains aligned to the b direction. Utilizing nanofabrication techniques, we are able to exfoliate and fabricate thin layer of fieldeffect transistors based on these materials and observe their transport behavior at high magnetic field and low temperature. 
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